MongoDB Atlas is the name of the cloud service run by MongoDB themselves, with which Amazon DocumentDB competes. https://www.mongodb.com/cloud/atlas
Not OP, but that wouldn’t necessarily be a surprise. As customers make product requests to AWS they can be tapped to test upcoming launches - anything from pre-release testing to very early alphas.
I can confirm that this is very much a thing that they do. We have an account manager able to bump feature requests over to the appropriate product managers, and have been involved in pre-release testing of features that we expressed interest in.
As others have said we had MongoDB Atlas. And it is basically mongodb ran in aws with a pretty interface to do basic things like whitelist ips and another such functions.